The present invention relates to an extrusion screw for thermoplastic matter. An extrusion screw for plastic matter is generally based on the principle of the Archimedes' screw. It comprises, in the classical and known concept, the following characteristics, given in order to improve understanding and for a more concise presentation in FIGS. 1 and 3.
In FIG. 1, the following classical characteristics are illustrated:
the angle of inclination or the helical angle of the thread, represented as .alpha.
the pitch AB of the thread, it is also equal to tangent .alpha.
the outside diameter CD of the screw,
the inside diameter or the diameter of the core, designated EF or GH,
the head T of the thread
the whorl MO
the width of the thread or width L of the whorl
the effective area of the screw is given by the area PQRX.
In FIG. 2, the following classical characteristics are illustrated:
V the empty space or cavity of the thread;
T the head of the thread
PQRX the effective area of the screw
S the width of the crest of the thread
QR the width of the root of the thread
.gamma. the angle formed by the flank of the thread and the longitudinal axis of the screw, in upstream direction
.delta. the angle formed by the flank of the thread and the longitudinal axis of the screw, in downstream direction.
The length of a classical extrusion screw depends on the material to be transformed and on the shape of the profile to be obtained by extrusion. Typically, it varies from 12 to 53 times the outside diameter CD.
The aim of an extruder screw is threefold, being:
to transport the plastic matter from a supply hopper where it is introduced, to a die where the matter assumes the profile given by the latter.
to bring about homogeneity by friction against the cylinder of the body of the extruder at the same time as its fluidification, by elevation of the temperature,
to compress all the matter.
To this end, a classical screw comprises at least three different zones, distinguished by a thread profile optimized so as to meet each of these requirements. Usually, the first section of the screw (first 1/3) is designed to advance the material without perceptible modification of the physical characteristics.
The central section (second 1/3) is designed to initiate the changing of the physical state by heating, i.e. the transition from the solid state to the molten state, of the thermoplastic matter.
The mechanism of the melting of thermoplastic matter in an extruder is complicated, and the melting is far from homogeneous.
The third section of the screw (third 1/3) is designed so as to accentuate the pressure, either by a reduction of the depth of the threads or by an increase in the pitch of the thread. That portion of the mixture which is still solid is broken up into small particles which intermix with the already molten phase at the same time as the fluid mixture is subjected to pressure which is ensured to raise it to the desired rate of compression.
The quotient of the effective area PQRX at the entry of the screw to the effective area at the exit is called the "rate of compression". It is obvious that a far higher rate of compression is required if a material in powder form is transformed than if the material is in granular form since, at a constant mass, there will, in the first case, be a far more substantial volume, owing to a lower volumetric mass (or apparent density). It is useless to attempt to increase the rate of compression by increasing the speed of rotation of the screw. This frequently corresponds only to an additional energy consumption, which may lead to an upset in the various temperatures of the extruder, as a result of overheating due to the too intense mechanical working.
In the prior art, the rate of compression and the length of the screw are not always adequate to ensure the good homogeneity of the material. Frequently other modifications in the concept of screws have been effected, for example, the addition of mixer fingers to one or more screw threads, or the use of a curve in one or more threads, or the use of a multiple of engaging screws, or even providing the screws with a variety of different successive components. All these modifications and additional devices are costly, cumbersome and easily damaged.